The present invention relates to machinery involving the conduction of electrical current between parts moving relative to each other, more particularly to methods and devices for effecting or facilitating such electrical conduction.
Various kinds of motors, generators and other electrical apparatus require the conduction of electricity between two relatively moving parts. Such mechanical arrangements usually involve the conduction of current between a stationary part (stator) and a rotating part (rotor). A device known as a “brush” or “current collector” is normally used for making sliding contact between stationary and rotating parts so as to conduct electrical current therebetween.
Depending on the particular machinery, a brush can be used to conduct current in either direction (i.e., either from the stationary part to the rotating part, or vice versa), and can be fixed with respect to either the rotating part or the stationary part. Among the desirable qualities of a brush are high current-carrying capacity (e.g., in terms of capability of carrying a high amount of current per unit area of the interface between the brush and the surface contacted thereby), low friction, and high wear resistance. Current collection brush technology has grown in interest with the advent and continued development of homopolar machine technology, particularly in the realm of homopolar motors (which operate on direct current) such as those that are currently envisioned for naval ship propulsion.
Conventional brushes include solid carbon brushes, copper fiber brushes and liquid metal brushes. The majority of brushes currently used are of the solid carbon variety. Solid carbon brushes provide limited power densities due to their characteristically small number of contact spots. In addition, solid carbon brushes tend to have a short life and to produce conductive wear debris, resulting in frequent brush replacement and frequent machinery cleaning and associated high maintenance costs. Generally speaking, as compared with solid carbon brushes, copper fiber brushes are considered to afford superior performance; however, copper fiber brushes are currently expensive to produce and can support only moderate current densities. It is generally believed that liquid metal brushes are capable of supporting very high current densities, but more research is needed in this area because of problems concerning stability and reactivity.
A conventional current collection assembly includes a brush and a “holder” (for the brush) as two separate components that are attached to each other. The holder is also attached to either the stationary part or the rotating part of the machinery. Soldering is normally implemented to achieve attachment between a brush and a holder. Small voltage drops are associated with solder joints, which can thus adversely affect performance. Moreover, solder joints are prone to mechanical failure.